(1) Field of the Invention
The present invention relates to ZrO.sub.2 --Y.sub.2 O.sub.3 zirconia ceramics having high strength and high resistance against deterioration of strength due to use for a long period of time within a specifically limited temperature range, and a method for producing the same.
(2) Description of the Prior Art
There have hitherto been known, as ZrO.sub.2 --Y.sub.2 O.sub.3 zirconia ceramics, fully stabilized zirconia ceramics consisting only of cubic phase and partially stabilized zirconia ceramics consisting of cubic phase and monoclinic phase, both of which are used as a heat resistant material and a solid electrolyte and the like. The fully stabilized zirconia ceramics are stable within a temperature range of from room temperature to about 1,500.degree. C. and show very little deterioration of strength due to use for a long period of time. However, the fully stabilized zirconia ceramics are poor in strength, and, for example, are apt to be very easily broken due to thermal shock during use as a solid electrolyte for an oxygen sensor used for detecting oxygen concentration in the exhaust gas of automobiles and the like. The partially stabilized zirconia ceramics consisting of cubic phase and monoclinic phase are higher in strength and thermal shock resistance than the fully stabilized zirconia ceramics. However, they have such serious drawbacks that the partially stabilized zirconia ceramics deteriorate very noticeably in their strength due to the lapse of time within a specifically limited temperature range of from 200.degree. C. to 300.degree. C. When such zirconia ceramics are used for a long period of time at a temperature within the above described temperature range, a large number of fine cracks are formed on the surface of the ceramics to make the ceramics water absorptive and to deteriorate their strength, resulting in breakage of the ceramics.
The reason for breakage of the partially stabilized zirconia ceramics may be as follows. Although the partially stabilized ZrO.sub.2 --Y.sub.2 O.sub.3 zirconia ceramics consist of cubic phase and monoclinic phase at room temperature, they consist of cubic phase and tetragonal phase at the firing temperature of about 1,500.degree. C. Therefore, the crystal grains of tetragonal phase at the firing temperature change their crystal phase from tetragonal phase into monoclinic phase at about 500.degree. C. during the cooling from about 1,500.degree. C. to room temperature, and excess stress is caused due to the volume change caused by the phase transformation. Thus a large number of fine cracks are formed in the crystal grains and grow up therein during use for a long period of time at a temperature within the range of from 200.degree. C. to 300.degree. C., resulting in breakage of the ceramics. Furthermore, when the partially stabilized zirconia ceramics consisting of cubic phase and monoclinic phase are repeatedly heated and cooled between room temperature and about 800.degree. C., the ceramics exhibit different thermal expansion curves in the heating direction and cooling direction due to the phase transformation between the monoclinic phase and tetragonal phase caused at about 500.degree. C., that is, exhibit a so-called hysteresis curve. Moreover when the ceramics are heated from room temperature and then cooled to the original room temperature, the ceramics have different dimensions before and after the heating, and therefore the ceramics can not maintain accurate dimensions.